Passive and Active Components, the Diode, and the Zener Regulator

The junction physics behind this topic is derived in Physics (GAPHT121) Module 3, and the breakdown mechanisms and Zener characteristic in its Module 4. This topic covers the same devices from the circuit side — what they do and how to use them — rather than repeating why they do it. Where the exam asks "explain the working", the physics is what earns the marks, so it is worth reading both.

Passive and active components

Passive components neither generate energy nor control the flow of current with a third terminal. They can only dissipate or store: resistors, capacitors, inductors, transformers.

Active components can amplify, or control a large current with a small one, and require a power supply to work: diodes, transistors, FETs, integrated circuits, and all sources.

The dividing line worth stating is control. A resistor's behaviour is fixed by its construction; a transistor's is set moment by moment by its input. That is what makes amplification and switching possible, and it is why every signal-processing circuit needs at least one active device.

Note the diode's awkward status: it has only two terminals and cannot amplify, yet it is classed as active because its resistance is non-linear and it can rectify. Different textbooks argue about this; KTU's syllabus lists it among electronic devices, which is the answer to give.

The PN junction diode, from the circuit side

A junction of p-type and n-type semiconductor. Forward biased (p positive) it conducts; reverse biased it blocks.

The V-I characteristic has three regions:

1. Forward, below the knee. Almost no current until roughly 0.7 V for silicon, 0.3 V for germanium.
2. Forward, above the knee. Current rises very steeply — a decade per 60 mV — so the voltage is nearly constant at $V_F$ regardless of current.
3. Reverse. A tiny, near-constant reverse saturation current $I_0$, until breakdown.

The practical model. For circuit work, treat a conducting silicon diode as a constant 0.7 V drop, and a non-conducting one as an open circuit. This is accurate enough for almost every calculation in this course, and it is what makes hand analysis possible.

The consequence that catches people out: a diode never has a resistance in the ohmic sense. Its dynamic resistance $r_d = V_T/I \approx 26/I(\text{mA})\ \Omega$ falls as current rises — about 26 Ω at 1 mA and 2.6 Ω at 10 mA.

Applications: rectification, clipping and clamping, freewheeling across inductive loads, and protection against reverse polarity.

Breakdown, and the Zener regulator

Zener breakdown occurs below about 5 V in heavily doped junctions, where the depletion region is thin enough for the field to strip electrons directly from bonds. Avalanche breakdown occurs above about 7 V in more lightly doped junctions, where accelerated carriers knock others free by impact and multiply. Their temperature coefficients have opposite signs, which is why diodes near 5–6 V are the most stable.

The regulator circuit. A series resistor $R_S$ from the supply, with the Zener reverse biased across the load. In its breakdown region the Zener holds $V_Z$ almost constant over a wide current range, so the load sees $V_Z$ while $R_S$ absorbs the surplus.

Two limits must both be respected, and they occur at opposite extremes of the operating range:

• Minimum Zener current (typically 5–10 mA) must be maintained, or regulation collapses. Worst case: minimum input, maximum load.
• Power rating $P = V_ZI_Z$ must not be exceeded. Worst case: maximum input, no load, when the Zener carries everything.

A design checked at only one extreme passes on paper and fails on the bench.

Its limitation. A shunt regulator wastes the surplus as heat, so efficiency is poor and it suits only low currents. Higher-power supplies use a series-pass transistor or a switching regulator, with the Zener demoted to providing the reference voltage.